Galaxy S3 I9300 users are in for a special treat, as the first official test firmware based on Android 4.2.1 Jelly Bean has been leaked online by SamMobile. Though the beta firmware is not yet officially available over Samsung KIES or Over-The-Air (OTA), discerning users can try a hand on the latest Jelly Bean firmware using the manual installation method with this guide.

As this is a private beta firmware for developers, interested users can get a glimpse of what to expect from Samsung in their upcoming software updates. The installation of this firmware makes use of ODIN flash tool and hence installation of ClockworkMod (CWM) recovery is only optional (required for installing third-party root apps, custom ROMs and latest system tweaks).

NOTE: As this is an unbranded firmware, it can be installed on any Galaxy S3 I9300 (irrespective of the device's carrier). This, despite the fact that the firmware is officially meant for Middle East region only.

SamMobile assures that they have fully tested the firmware and it should work as good as any other official firmware. As this is a private test firmware, it may harbour a few untested bugs or broken features. However, any such bugs are expected to be fixed in the upcoming official public release.

Key Points to Note

Android 4.2.1 XXUFMB3 official test firmware and the instructions given in this guide are applicable to the international variant Galaxy S3 I9300 model only. It will not work on any other device. Verify your device's model number by navigating to Settings > About Phone.

Back up important data and settings on your phone by using one of the recommended custom recovery tools such as ClockworkMod (CWM) or TWRP, as it creates a complete image of the existing phone setup.

Make sure that USB Debugging mode is enabled on the phone by navigating to Settings > Developer Options (if the device is running Android 4.0 or later) or go to Settings > Applications > Development (if the phone is running Android 2.3 or earlier).

Ensure that your phone has at least 85 percent battery charge to prevent unexpected device shutdown (due to battery drain) while flashing the official test firmware.

Verify that the device is factory unlocked and not locked to a specific carrier before proceeding with the firmware installation.

The instructions provided in this guide are meant for reference purpose only. IBTimes UK will not be held liable if you damage or brick the device during or after firmware installation. Users must proceed at their own risk.

Step 3 -Power off your phone and boot into Download Mode. To do so, press and hold Volume Down and Home buttons together, and then tap Power button until the construction Android robot icon with a triangle appears on screen. Hit the Power button again to entry into Download Mode.

Step 4 - Launch ODIN on computer and connect your phone to it while your phone is still in Download Mode.

Step 5 - When the phone connects successfully, one of the ID: COM boxes turns yellow with the corresponding COM port number. This step may take some time. So, do not interrupt the process.

Step 6 - Choose the files that need to be flashed or installed on your phone, which can be found among the extracted files in Step 1.

Click the 'PDA' button and choose the file named 'CODE' or '.md5'.

Tap the 'Phone' button and choose the file named 'MODEM'. Ignore this step if there is no such file.

Hit the 'CSC' button and choose the file named 'CSC'. Ignore this step if such file is missing.

Click the 'PIT' button and choose the .pit file. Ignore this step if such file is absent.

Step 7 - In ODIN, enableAuto Reboot and F. Reset Time options. In case of Re-Partition option, check it ONLY if you chose a .pit file in the previous step.

Step 8 - Hit the Start button in ODIN and the firmware Installation process begins. It would take just a few minutes to complete.

Step 9 - Once the firmware installation is done, your phone will reboot. When the home screen appears, unplug the USB cable to disconnect your phone from computer.

Your Samsung Galaxy S3 is now successfully updated with Android 4.2.1 XXUFMB3 official test firmware. Go to Settings > About Phone to verify the firmware version installed.